Removal of electrodeposited metals



alloys which coatings but which does not result in attack of the Patented Mar. 13, 1945 2,371,529 REMOVAL OF ELECTRODEPOSITED a METALS William S. Loose,Midland, Mich., assignor to The Dow Chemical Compan poration oi Michigan y, Midland, Micl1., a cor- No Drawing. Application September 20, 1941,

Serial No. 411,698 I 3 Claims. (01. 204-146) This invention relates to a method of removing metal coatings from the surface of articles formed of:magnesium and magnesium-base alloys.

In most commercial electroplating-processes a certain small percentage of the plated articles are, for one reason or another, rejected on inspection as being imperfectly plated. It is customary, in the interests of economy, to put the rejected articles through a strippingoperation to remove the electroplated metaL'and then to return the stripped articles to the original process for the replating. As a result, there have been developed a considerable number of stripping baths which are eifective in removing electrodeposite'd metal coatings from articles formed of most common metals. Unfortunately, however, none of these baths is fully effective in removing electrodeposited metals from the surface of magnesium and magnesium-base alloys, and all are subject to the great disadvantage that they attack the magnesium seriously during the stripping'treatment.

An object of the invention, therefore, is to provide a method of stripping metal coatings, especially electrodeposited coatings, from the. surface of articles of magnesium and magnesium-base is fully effective in removing the base metal.

In the process ofthe invention, the metal-coating is stripped by immersing the coated article in an aqueous acidic solution of a fluoride and passing current through the article to cause it to function as anode in the solution for a time suflicient I to remove the-metal coating.

Although a variety of acidic fluoride solutions may be used as stripping baths, aqueous solutions of hydrofluoric acid, preferably those containing. 10 to 50 per cent by weight of the acid. or of mix- "tures of the alkali metal and ammonium fluorides acid, are

orbifluorides together-with a mineral especially convenient. Preferred baths are aque-- our; solutions containingi'rom about 10 to about 50 per cent by weight of hydrofluoric acid and a lesserproportion of one Ofthe mineral acidsz sulfuric, hydrochloric, hydrobromic. and

hydriodic acids. especiallythose baths'containing between about -1 and about 20 per cent by weight' of nitric acid or between about 0.25 and about 10 per cent by weight of hydrochloric acid. In genera], the proportion of mineral acid should be less than ma of the, hydrofluoric acid, since baths chloric acid. Current was passed through the m article as anode at a current density falling grad 1s conveniently but not necessarily at room temperature, and is made the anode in an electric circuit, the cathode being such as carbon or nicke Current is passed through the article at a density of 10 to 100 amperes per square foot at any convenient voltage, usually at least 2 to 3 volts or higher, for a time suff cient to remove the metal coating, usually for 10 to 30 minutes in the case of coatings of ordinary thickness. As the coating dissolves in the bath, the current passing at a given voltage gradually falls off; when the metal coating is entirelyremoved substantially no current flows in the circuit, presumably due to the formation on the article of a, very thin non-conducting film of magnesium fluoride. When this condition is reached, the article is removed from the solution, and is rinsed in water. It may then be at once replated, if desired. v

Among the metal coatings which have been successfully removed from magnesium articles are those of nickel, cobalt-and iron, which may in my Patent No. 2,313,- r

Example 1 A die-casting formed of- Dowmetal R (a I nesium-base alloy containing 9.0'per cent alumithick was immersed in 25 per cent aqueous hydro fluoric'acidcontaining 1.0 percent of hydroually'from about 100 toless square foot until the nickel than 1 ampere per plate was entirely removed. The articlewas then withdrawn from the bath and'rlnsed. The resulting surface clean, unpitted, and free of nickel.

- Example 2 was A die-casting formed or Dowmetal R and hav- 'ingon its surface an electrodeposited coating of nickel 0.0005 inch thick, with a, copper electroplated coating 0.0005 inch thereupon, was made the anode in a 2 0 per cent hydrofluoric acid solucontaining higher proportions ofstrong acid may at tlmes'tend to pit the magnesium surface.

. In practice, the plated magnesium article to-be tion at room temperature and, at a starting current density of 50-60 amperes per square foot for 30 minutes.

The composite metal electroplate stripped is immersed in the stripping bath, which any suitable material,=.

was entirely removed without any pitting oi the base metal. I g

Other-modes or applying the principle or the invention may beemployed instead of those explained, change being made as regards the details disclosed, provided the method recited in any of theiollowing claims, or the equivalent thereof,

be e ployed.

1 I claim:

1. A method according to claim 3 wherein the 3. In a method of removing electrodeposited coatings of a metal selected from the group consisting of iron, cobalt, and nickel from the surface of articles formed of magnesium and magnesium-base alloys, the steps which comprise immersing the article in a solution consisting ot water and from about 10to about 50 per cent by weight of hydrofluoric acid and a lesser proportion of a mineral acid selected from the class consisting of nitric, sulfuric, hydrochloric, hydro-' bromic, and hydriodic acids and passing current thrormh the article to cause it to function as anode in said solution at a current density between about 10 and about 100 amperes per square foot for a time sufficient to remove the metal coating.

WILLIAM S. LOOSE. 

